cex: don't display twice unifying examples if there is no color

It makes no sense, and is actually confusing, to display twice the
same example with no visible difference.

* src/complain.h, src/complain.c (is_styled): New.
* src/counterexample.c (print_counterexample): Display the unified
example a second time only if it makes a difference.
* tests/conflicts.at, tests/counterexample.at, tests/report.at: Adjust.
* tests/diagnostics.at: Make sure we do display the unifying examples
twice when colors are enabled.  And check those colors.

src/complain.h

@@ -36,6 +36,8 @@ void end_use_class (const char *style, FILE *out);
 /** Flush \a out.  */
 void flush (FILE *out);
 
+/** Whether there's styling on OUT.  */
+bool is_styled (FILE *out);
 
 /*-------------.
 | --warnings.  |